Estimation of disease severity

Learning Objectives

1. Know the difference between disease severity and disease incidence, and between accuracy and precision.
2. Compare relative advantages of two types of scales used for disease severity estimation: a logarithmic scale (Horsfall-Barratt) and a percentage scale.
3. Understand how the use of a scale can later affect data analyses. 

Time required
2-3 hr.

Materials needed

• Access to a computer with EXCEL®
• LB Learning Resource CD (also availabale on Internet)
• Paper and pencil for notes

Activity 1.  Estimating with a percent scale

• Go to the Disease Pro directory and run Disease.exe.
• Assess disease severity on 40 leaves using the percentage scale.
• Wipe all score cards (Menu 9).
• Go to Menu 1 and select 40 leaves with no actual percentage to be shown.
• Go to Menu 2 and select a disease, lesion size, and presence or absence of chlorosis. Write down your selection because you will use the same settings in the next step.
• Go to Menu 3 and start the simulation.
• Complete your evaluation and then go to Menu 7 to write your data to a file.  his file will be saved in the directory of DISEASE.PRO.
• Open the file that you recently created with Excel. If you can’t see your file, choose All Files in File of type. In the Text Import Wizard choose Delimited,  Next, Space, and Finish. Copy and paste your results into the appropriate  columns of in the Excel file on Estimating Disease Severity provided in the CD.
• Your data will be plotted in the Excel sheet.  What parameters can be used to evaluate the accuracy and the precision of the estimates.

Activity 2.  Comparing percent and logarithmic scales

• Repeat the exercise above but this time estimate disease severity with 
  the former “CIP” scale, which is a Horsfall-Barratt scale.  Apply, i.e., type in  Disease Pro, one of the “class” values from this scale (see it here or at the end of this activity) for each diseased leaf – don’t estimate the percent infection.  Be careful to use the same settings (disease, lesion size, and presence or absence of chlorosis).
• Go to Menu 7, write your data to a file, open it using EXCEL, and copy and  paste your results into the appropriate columns of Table 3 the file Estimating disease severity.xls , in the Misc. Resources directory.

Activity 3.  Analyze your results.

• After you paste your results in Table 2 and 3, several figures and analyses will   be automatically done.
• Break into two groups.   Use one card to answer each of the following 
  questions
• What patterns do you see in Figure 1? Is the distribution of the errors 
  random, or is there more variability in a certain range of disease severity?  Relate your observations to the Weber-Fechner law (you can read about it in resources).
• Notice in Table 3 that the classes estimated with the Horsfall-Barratt scale are transformed back to percentage. Give your opinion about this 
  transformation.
• Which scale enables more accurate estimates of disease severity? Which 
  enables more precise estimates? Why?
• How might the use of one of these scales (the Horsfall-Barratt vs. the direct  percentage) affect your analyses and interpretation of results?
• One representative from each group will present the results